The cooling system for an internal combustion system is typically cooled either by water (or a liquid coolant) or air. An air cooled engine is relatively economical to build because of a smaller number of components that are required. A water cooled engine typically employs a water pump to circulate water in the water jacket of the engine, a radiator to cool the water from the water jacket and a thermostat to regulate the flow of water in the water jacket to maintain the temperature of the cooling water at a prescribed level. Thus, a water cooled engine has the advantage of cooling the engine in a stable manner, but has the drawback of requiring a relatively large number of component parts. Therefore, a water cooled engine is usually more expensive to build and larger in size than a comparable air cooled engine.
In recent years, internal combustion engines known as controlled autoignition (CAI) engines have come to be realized as an attractive option because of the lean stoichiometries at which such engines can operate, and the associated low NOx emissions. For a CAI engine to operate in a satisfactory manner, it is desirable to quickly raise the temperature of the coolant to a prescribed temperature at the time of startup while preventing the coolant temperature from becoming excessive. Therefore, a water cooled engine that allows an accurate temperature control may be suited for a CAI engine, but has the drawback of requiring a significant time period for the engine to warm up so that the engine may not operate in a stable manner for a considerable time period until the engine fully warms up.
Based on such considerations, the inventor of this application has recognized that the boiling cooling system may be a suitable cooling system for a CAI engine because the boiling cooling system requires a relatively small number of components, and can reach a steady state in a relatively short period of time. In the boiling cooling system, because the upper limit of the coolant temperature is determined by the boiling point of the coolant, the need for a thermostat or other temperature control devices is eliminated. However, in order to achieve a compact CAI engine design that is suitable for a small general purpose engine, the cooling system is required to be designed as a compact unit.
In a known boiling cooling system disclosed in JPH05-47352U, a gas/liquid separator that is normally used in a boiling cooling system is omitted, and the radiator is tilted rearward so that the gas/liquid separation may take place within the radiator. However, a pump is required to recirculate the coolant. Therefore, there is a demand to further simplify the boiling cooling system for internal combustion engines.